A number of mechanisms are known for allowing transient poration of cells for the introduction of therapeutic agents. Recently, this has included a suite of optical methods, which add the potential of sterility, reconfigurability and single cell selectivity. For example, high repetition rate (˜80 MHz) femtosecond laser pulses of low average power (<100 mW) can be used to create low density plasmas at a focus that react with the cell's lipid bilayer causing transient pores to be generated, see Vogel, A., Linz, N., Freidank, S. and Paltauf, G., Phys Rev Lett 100 (3), 0381021-0381024 (2008). They are suitable for membrane poration of cells achieving high transfection efficiency of up to 80% with an acceptable cell viability of ˜80%, as described by Tirlapur, U. K. and Konig, K., Nature 418 (6895), 290-291 (2002) and Tsampoula, X. et al., Appl Phys Lett 91 (5), 053902 (2007). However, this approach requires the precise positioning of the laser focus on the cell membrane and is unsuitable for a high-throughput transfection system.
Cavitation bubbles can also play an important role in plasma membrane poration. It is known that acoustic bubbles oscillated by ultrasonic irradiation (insonation) can lead to enhanced membrane permeabilization (sonoporation) of cells. This is described in the art, see for example: Feril, L. B. et al., Ultrasound Med Biol 29 (2), 331-337 (2003); Miller, D. L. and Song, J. M., Ultrasound Med Biol 29 (6), 887-893 (2003); Honda, H., Kondo, T., Zhao, Q. L., Feril, L. B. and Kitagawa, H., Ultrasound Med Biol 30 (5), 683-692 (2004), and Prentice, P., Cuschierp, A., Dholakia, K., Prausnitz, M. and Campbell, P., Nat Phys 1 (2), 107-110 (2005). When cavitation occurs near a rigid boundary such as a cell membrane, the bubbles tend to collapse asymmetrically, often forming high speed liquid jets directed toward the wall that can cause localized membrane poration, thus allowing the uptake of molecules by these cells (Benjamin, T. B. and Ellis, A. T., Philos Tr R Soc S-A 260 (1110), 221-245 (1966)). However, this approach often activates multiple cavitation bubbles, which lead to non-uniform and sporadic molecular uptake that lacks refined spatial control.
Cavitation bubbles can be produced by the optical breakdown or ablation of an absorptive medium (see Barnes, P. A. and Rieckhof, K. E., Appl Phys Lett 13 (8), 282-284 (1968) and Bell, C. E. and Landt, J. A., Appl Phys Lett 10 (2), 46-48 (1967)). Of the approaches devised to date, membrane permeabilization of cells has been achieved at the expense of a significant amount of cell lysis owing to the relatively high breakdown threshold of the absorptive substances used in this procedure, e.g. water (present in the medium) or silica (glass coverslips) resulting in a much larger cavitation bubble (millimeters in diameter) compared to cell size (tens of micrometers) See Hellman, A. N., Rau, K. R., Yoon, H. H. and Venugopalan, V., J Biophotonics 1 (1), 24-35 (2008); Rau, K. R., Quinto-Su, P. A., Hellman, A. N. and Venugopalan, V., Biophys J 91 (1), 317-329 (2006), and Soughayer, J. S. et al., Anal Chem 72 (6), 1342-1347 (2000).